Couldn't you just walk to the back of the Visiting Center (around the outside) and use the ladder to get on the landing pad?
The only way to get to that ladder is past the guard. This is how I did it actually. I just ran past him and climbed up. The guards freaked and all started shooting, though they couldn't get an angle on me once I was up on the roof. There was one guard up there who I chopped in the face with a meat cleaver. Shortly thereafter, whooping like a savage, I jumped off the back of the roof and ran off down the road. The pursuing soldiers quickly gave up chasing me as I was too crazy and fast for them.
As soon as the first gun was drawn, the President's honor guard began charging the helipad, taking the President with them. Once they got there and ascertained that no assailants were still on the tower, they all jumped in the vertibird and got the hell out.
I did mention that I planted the bomb when the roof guard was dead and no one could really see what I was doing, didn't I?